The present invention proceeds from a gas turbine which in the simplest case has a diagram as reproduced in FIG. 1. The gas turbine 10 of FIG. 1 comprises a compressor 12, a combustor 13 and a turbine 15. Combustion air is drawn in, via an air intake 11, and compressed by the compressor 12. The compressed air is introduced into the combustor 13, where it is used for the combustion of a fuel 14. The hot gas produced is expanded in the downstream turbine 15, performing work, and leaves the turbine 15 as exhaust gas 16.
Modern (stationary) industrial gas turbines (IGTs) are generally configured with annular combustors. In the case of generally smaller IGTs, the combustors are embodied as what are termed “can-annular combustors”.
In the case of an IGT having an annular combustor, the combustion space is bounded by the sidewalls and the inlet and outlet planes of the hot gas. The combustor sidewalls are in this case either of segmented construction, being composed of shell elements, or are constructed as whole shells. When using whole shells, it is necessary for assembly to have a separating plane by means of which the upper part can be removed, in order for example to assemble or disassemble the gas turbine rotor. The separating plane accordingly has two separating plane weld seams which are, for example, at the level of the machine axis (at the 3 o'clock and 9 o'clock positions). The lower and upper half-shells must inter alia be convectively cooled.
Function of the Combustor Shells
The combustor shells (“combustor transition duct”) have the following functions:                They seal off two plena/chambers.        They also have to seal with respect to one another (assembly by means of a separating plane, generally at the 3 o'clock and 9 o'clock positions).        They are of rotationally symmetric design, with the exception of the separating plane.        They must be guided into/onto one another in the separating plane during assembly of the combustor half-shells.        The combustor inner shells or the inner combustor shells must be guided into one another “blind” at the separating plane (no access for a visual inspection of the connection plane as this plane is covered by the combustor inner shells).        They should not have to take up any axial or radial forces.        They may, but need not necessarily, be designed to be self-supporting (no supporting structure).        They must have (considerable) axial and radial freedom of movement, in particular during transient operative states.        They must be thermally stable (creep strength/fatigue strength).        Harmonic oscillations should be damped where possible (support for the shells).        
FIG. 2 shows a section, comprising the combustor, of an exemplary gas turbine having an annular combustor. The outlet of the compressor 12 with its guide vanes and rotor blades is shown here on the right; on the opposite side is the inlet region of the turbine 15 with its guide vanes and rotor blades. Between the compressor outlet and the turbine inlet region there is a rotor cover 25 which surrounds the rotor 17. The inlet region of the rotor cover is configured as a compressor-diffuser having a flow cross section which increases in the flow direction and through which the compressed air flows into a plenum 18 which surrounds the annular combustor 13. The combustor 13 consists of an inner combustor shell 20a and an outer combustor shell 20b. Inner and outer cooling jackets 19a and 19b are arranged with separation on the respective outer sides of the combustor shells 20a,b and form with the associated combustor shell respectively an inner cool air feed 21a and an outer cool air feed 21b. 
Air from the plenum 18 flows through these cool air feeds 21a,b into the inlet region upstream of the combustor 13, in which inlet region the actual burners 22 (in the present case what are termed double-cone burners) are arranged. The air fed in through the cool air feeds 21a,b enters the burners 22 on one side, where it is mixed with fuel. On the other side, air 24 enters the combustor directly through the rear wall 23 of the combustor 13. What is important for smooth operation of the gas turbine is the transition region, marked in FIG. 2 with a dotted circle and the reference sign A, between the combustor 13 and the turbine 15.
In operation, the inner and outer shells of the combustor are subject to high thermal and mechanical load. The material strength properties of the shells are very dependent on temperature. In order to keep this material temperature below the maximum permissible material temperature, the shell elements—as already described in conjunction with FIG. 2 and the cooling jackets 19a,b shown therein—are convectively cooled.
The shaping and the high thermal loading close to the turbine inlet require in particular in this region a constantly high heat transfer also on the cool air side. Both combustor shells reach, before the combustor is ignited, at least the temperature of the compressor outlet air. Once the burners are ignited, the metal temperature of both combustor shells increases further.
On account of the high metal temperature of the combustor shells, the shells expand axially and radially (see expansion direction 33 in FIG. 4). This expansion is easily measured in particular at the interface at the inlet of the turbine (inner and outer platform of the 1st guide vane row). This expansion occurs continuously and over a determined time period, during the startup process and in the event of changes in load on the gas turbine. The same process occurs in reverse during cooling down of the combustor (shrinkage).
In practice, it has now come to light that, by means of the type and configuration of the transition between combustor and turbine inlet as is described here, there result undesired abrasion marks or abrasion which must be avoided. As a consequence of the accompanying wear during operation of the gas turbine, the functionality is affected. Moreover, the lifespan is also affected or reduced. Finally, increased cost in reconditioning the machine is also to be expected.